1. FIELD OF THE INVENTION
The present invention relates generally to apparatus and method for making containers from thermoplastic materials. More particularly, the present invention relates to apparatus and method for blow molding a pair of containers from a parison tube of extruded thermoplastic tubing, and for correcting curvature in the parison tube that causes parison tubes to be pinched between the mold halves rather than being enclosed within the neck and moll cavity and that results in flashing of plastic material along the pinched portion and non-uniform wall thicknesses in finished containers.
2. DESCRIPTION OF THE PRIOR ART
It is common practice to form containers from thermoplastic materials by extruding a thermoplastic tubing from a crosshead, forming a parison tube by severing a portion of the thermoplastic tubing, placing the parison tube into a blow mold, and then blow molding the parison tube to the contour of the blow mold.
Futher, it has been common practice to place a number of blow molds on a rotating table in such a fashion that each blow mold opens and straddles the thermoplastic tubing as the thermoplastic tubing is being extruded from the crosshead, to sever a parison tube from the tubing, to grasp the parison tube between the mold halves, and to blow a container from the parison tube.
An example of this prior art is the subject matter of U.S. Pat. Nos. 2,515,093 and 2,579,390 which were issued to Mills. In these prior art patents, a horizontally disposed table rotates about a vertical axis; and the thermoplastic tubing is extruded horizontally towards the blow mold.
Typically, the thermoplastic tubing is extruded continuously, and the blow molds move continuously around the pivot axis of the table to which they are mounted. As each blow mold is aligned with the thermoplastic tubing, the blow mold is closed, a parison tube is severed from the thermoplastic tubing, the parison tube is grasped by a holding portion that is adjacent to the crimped-off end of the parison tube, a body portion of the parison tube is enclosed in a body cavity of the blow mold, and a neck and moil portion of the parison tube is enclosed in a neck and moil cavity of the blow mold.
As a given blow mold progresses around the pivotal axis, air is introduced inside the parison tube, blowing the parison tube to the contour of both the body cavity and the neck and moil cavity. Then the blow mold is opened, and the finished container is removed.
Subsequently, both the holding portion and the moil are removed from the finished container, the neck of the container is reamed, and the seal face of the container neck is smoothed by the application of heat.
Typically, the thermoplastic tubing is extruded to the largest diameter that will fit into the blow mold; so the outside diameter of the thermoplastic tubing will be slightly smaller than the diameter of the neck and moil cavity.
One major problem in rotary blow molding equipment is precise alignment of the parison tube into the mold cavity. The parison tube must be aligned concentrically in the mold to assure that, upon inflation of the parison tube, it will expand uniformly.
If the parison tube is not positioned properly in the mold halves, upon closing, the mold halves may nip one side of the parison tube. If one side of a parison tube is nipped between the closing mold halves, the blown container will have flash on one side, and there will be an uneven distribution of the parison material.
Uneven distribution of the parison material will result in a finished container that is thin on one side and that may rupture at some later time; or uneven distribution of the parison material may result in the container bursting during the blow molding operation.
Proper alignment of the parison tube in the mold halves is highly dependent upon straightness of the parison tube. If there is bowing or curvature in the parison tube, one side of the parison tube will be pinched between the mold halves at the neck and moil cavity, as opposed to the parison tube being enclosed inside the neck and moil cavity.
As described above, a portion of the parison tube being pinched between the mold halves results in unequal wall thickness in the finished container with the possibility of rupturing in use, or even in the container wall rupturing during the blow molding process. Thus, it is important to avoid, to minimize, or to correct any bowing or curvature in the parison tube.
In U.S. Pat. No. 3,764,250, Waterloo recognized the problem of curvature in the thermoplastic tubing and provided apparatus for correcting the problem.
Waterloo devised blow mold apparatus in which the blow molds are mounted onto a vertically disposed table that rotates about a horizontal axis. The thermoplastic tubing is extruded upwardly; and the blow molds engage the thermoplastic tubing at a rotational position wherein each blow mold moves upwardly and away from the crosshead, with the objective of the blow mold pulling and straightening the thermoplastic tubing as it is being extruded.
On the other hand, there may be a number of factors that are responsible for the bowing of newly extruded plastic tubing during the process of blow molding such tubing into containers and the like. One particular factor that seemingly plays a role in the bowing or curving of such tubing is the movement of the molds in a blow molding machine of the rotary type.
That is, the molds are moving continuously in a circular path as the mold halves grasp the parison tube near the ends thereof. As the ends of the parison tube are being grasp by the mold halves, the mold halves are moving along the circular path. Thus, because of the continuous movement of the molds in a circular path, the mold halves impart an acceleration to the parison tube as the mold halves change the velocity of the parison tube to be in the same direction as, and equal to, the mold halves.
Also, as can be recognized, the parison tube is hot and deformable upon being clamped off by flattening the ends therof. It is believed that this clamping and flattening of the ends of the parison tube result in a lengthening of the parison tube; and, since the ends of the parison tube are restrained between the partially closed mold halves, the excess length results in a bowing or curving of the parison tube.
Also, curvature in a parison tube may be due to uneven flow in the crosshead, so that the thermoplastic tubing has a curvature as it is being extruded; or the thermoplastic tubing may develop a curvature due to gravity, such as in the apparatus of Mills wherein the thermoplastic tubing is extruded horizontally.
It appears that the apparatus of Waterloo might be effective in correcting curvature resulting from uneven flow in the crosshead, or in correcting curvature resulting from extrusion of the thermoplastic tubing along a horizontal axis. However, his apparatus would not be effective in correcting curvature due to lengthening of the parison tube due to clamping of the ends of the parison tube.
The acceleration on the parison tube is generally along the axis of the extruded tubing in the apparatus of Waterloo because the axis of extrusion is substantially tangential to the rotary movement of the closing mold halves.
While the apparatus of Waterloo minimizes acceleration loads that are transverse to the parison tube, and while this arrangement would seem, at first glance, to minimize curvature due to transverse acceleration of the parison tube, it is believed that the longitudinal acceleration that is imparted to the parison tube lengthens the parison tube, between the clamped-off ends thereof, and results in stretching longitudinal portions of the parison tube that are attempting to accelerate other longitudinal portions of the parison tube. Then, this lengthening of the parison tube between the clamped-off ends results in bowing or curvature of the parison tube.
Further, even though the apparatus of Waterloo does minimize acceleration loads on the parison tube that are transverse to the extrusion axis, it will be shown subsequently that minimizing transverse acceleration loads is a disadvantage, when seen in the light of the present invention.
The problems of curvature in the parison tube have become increasingly severe because of an increase in the rates at which thermoplastic material may be extruded from crossheads.
One way to increase the ability of blow molding apparatus to keep abreast with increased extrusion rates, and to increase blow molding productivity, is to increase the velocity of the blow molds as they are rotated around their pivotal axis.
This increase in rotational velocity of the blow molds has increased the required closing velocity of the blow mold, which has increased the clamping velocity, thereby increasing the lengthening and resultant curvature of the parison tube. This increase in closing velocity of the blow mold also has decreased the time in which the curvature of the parison tube may be corrected by being deflected into the neck and moil cavity.
Another way to increase the ability of blow molding apparatus to keep abreast with increased extrusion rates and to increase blow molding productivity is to use a blow mold that includes cavities for two separate containers. Blow molds for extruding containers in pairs is taught by Ruekberg in U.S. Pat. No. 3,456,290.
Blow molding containers in pairs (so-called "logged" blow molding) results in a parison tube that is approximately twice as long as the parison tube for a single container, thus increasing curvature problems in the parison tube whether caused by curvature in the tubing as extruded, by gravity forces on thermoplastic tubing that is extruded horizontally, by clamp-off, or by acceleration forces, whether longitudinally along the extrusion axis or transversely to the extrusion axis.
The severity of the problem can be seen in fact that, quite often, the parison tube will have a length of at least ten diameters when two containers are formed simultaneously in a single blow mold by a single inflation.
In the past, it has been more practical to extrude the thermoplastic tubing vertically, as opposed to horizontally, for blow molding thermoplastic containers, because of gravity-caused deformation of the tubing.
However, vertical extrusion of the thermoplastic tubing has meant that the blow molds must rotate in a vertical plane around a horizontal axis; and this arrangement has resulted in the height of the blow mold apparatus being excessive, even to the extent of limiting machine capacity because of the height of factory ceilings.
With the economic necessity of blow molding containers in tandem, and with the doubling of the length of the parison tube when blow molding containers in tandem, the problems of deflection of horizontally extruded tubing have increased tremendously.
And yet, with this economic pressure to blow mold thermoplastic containers in tandem, there has been a simultaneous economic pressure to build larger and faster blow molding machines that include a larger number of blow molds. A larger number of blow molds means that the blow molds are disposed around a larger circumference; and a larger circumference, for horizontally extruded tubing, means greater height of the blow mold apparatus.
In summary, the advantages of being able to correct curvature in the parison tube include: the ability to successfully blow mold containers in tandem, the ability to blow mold containers in tandem with apparatus in which the thermoplastic tubing is horizontally extruded and the height of the apparatus is minimized by rotating the blow molds around a circle in a horizontal plane, improved quality of the containers, and a lower rejection rate in container production.
Thus an objective of the present invention is to provide apparatus and method for reducing the curvature in a parison tube and for enclosing the parison tube in the neck and moil cavity of the blow mold without pinching the parison tube between the halves of the blow mold.
Another object of the present invention is to provide apparatus and method for orienting curvature of a parison tube in a predetermined plane and in a predetermined direction within the plane, and for applying a straightening force to the parison tube that is generally in an opposite direction in the same plane.
Another object of the subject invention is to provide means for assuring the concentricity and straightness of a plastic parison tube within a contoured mold, especially in a tandem mold.
Still another object of the present invention is to provide means for aligning and positioning a parison tube to maintain proper alignment within a mold, and thereby to insure even and uniform expansion of the walls of the parison tube upon inflation.
Still another object of the present invention is to provide means for adjusting the length of the plastic parison tube during its advance from one station to another station, so that bowing or curvature in the parison tube is reduced.
Finally, an object of the present invention is to correct curvature in parison tubes so effectively that containers may be blow molded in tandem using apparatus in which the maximum height of the apparatus is minimized by extruding the thermoplastic tubing horizontally and transporting the blow molds in a circular path of a horizontal plane.